1. Field of the Invention
The present invention relates to a zinc oxide (ZnO)--vanadium oxide varistors comprising a blend of metal oxide additives and having improved nonlinear current-voltage characteristics, more particularly to a ZnO--vanadium oxide varistor able to be sintered at a relatively low temperature and having a lower breakdown voltage and leakage current and a higher value of the nonlinear coefficient.
2. Description of the Prior Art
Varistors are resistors with resistance varying in voltage in a nonlinear relation. Varistors have high resistance and are good resistors when loaded with voltage below the critical voltage. The resistance of the varistor abruptly decreases and the electric current through the varistor will greatly increase when voltages are higher than the critical voltage. In other words, varistors possess the ability to absorb surges, reduce overload voltage to a safe level and prevent electric components from being damaged by surges.
As to the electric characteristic of varistors, the value of voltage (V) selected to give a 1 mA current through the varistor is called the breakdown voltage (V.sub.B). The leakage current (I.sub.L) means the electric current at the voltage of 0.8 V.sub.B. The breakdown characteristics of varistors are very important in application. The relationship between voltage and electric current is expressed by the equation of EQU I=k V.sup..alpha.
wherein I represents the electric current through the varistor, k is a constant, V represents the voltage applied across the varistor, and .alpha. is a nonlinear coefficient. Based on the above simple equation, the a value of the nonlinear coefficient can be determined by the values of the voltage and the electrical current.
The technique of producing zinc oxide varistor has been in development for several years. Bi.sub.2 O.sub.3 has been used as an elementary additive up to now. When ZnO and Bi.sub.2 O.sub.3 powders are sintered, Bi.sub.2 O.sub.3 will segregate at the ZnO boundaries as an intergranular phase and form an electronic conduction barrier between ZnO grains, so that the ZnO ceramics exhibit nonohmic behavior. Several other oxides are added into general commercial varistor products to enhance the nonohmic effect, such as to improve intergranular barriers and microstructure. The zinc oxide varistors produced by conventional methods of bulk ceramics have excellent nonlinear voltage (V)--current (I) characteristics. That is, based on the above equation of I=k V.sup..alpha., the nonlinear coefficient .alpha. is more than 50. The varistors are widely applied since the electrical stability and the ability to absorb surges are good.
Recently, electrical appliances gradually have become smaller and integrated circuits are frequently used. General bulk varistor cannot be directly applied because of high working voltages. In order to protect the integrated circuit elements from being damaged by surges, varistors with low working voltages are necessary, the multilayer chip varistors with low working voltages are particularly important. The advantages of the multilayer technique for producing varistors are for instance: (1) the volume of varistors is reduced for subsequent surface mount technology; and (2) the breakdown voltage ranges from several voltages to dozens of voltages simply by controlling the thickness of the ceramic layer. In addition, the design of the multilayer can achieve the purpose of increasing electrode area. As compared with other methods of producing low voltage varistor, such as thin foil method, thick film method, and seed grain method, multilayer method can produce in a large scale, have good reproducibility and higher surge absorption per volume.
Traditional ZnO--Bi.sub.2 O.sub.3 system varistors are obtained by sintering at a temperature above 1100.degree. C. The internal electrodes within the multilayer varistor should be the expensive palladium (Pd) or platinum (Pt) material due to their high melting point. However, it is noted that when the ZnO--Bi.sub.2 O.sub.3 system and the palladium internal electrode are sintered at high temperatures, Bi.sub.2 O.sub.3 and the palladium internal electrode produce a solid solution reaction and result in damage. In U.S. Pat. No. 4,290,041, Utsumi et al. uses borosilicate-lead-zinc (i.e. Pb--B--Zn--Si) glass as the principal additive to replace the original Bi.sub.2 O.sub.3 additive. But a high sintering temperature, for instance more than 1100.degree. C. is still necessary in Utsumi's method. Therefore, it is inevitable to use the expensive palladium or platinum material as internal electrodes which will keep the manufacturing cost high.
Based on the above description, the principal additives used in commercial varistors are classified into three series: bismuth based additives (Bi.sub.2 O.sub.3), praseodymium based additives (Pr.sub.6 O.sub.11) and borosilicate-lead-zinc (Pb--B--Zn--Si) glass. In order to solve the problems of the above mentioned three series, the present invention provides a vanadium based additive (V.sub.2 O.sub.5) used in ZnO varistors. The varistors of the present invention not only can be sintered at a lower temperature of 900.degree. C. due to the addition of V.sub.2 O.sub.5, but also have excellent nonlinear current-voltage characteristics by the incorporation of several metal oxides. Sintering at a relatively low temperature is important for applications because the ZnO--V.sub.2 O.sub.5 system can be cofired with a silver inner-electrode (m.p. 961.degree. C.) rather than using the expensive palladium or platinum material.